I agree with most of what has been said. I have no factual evidence but I've made changes based upon my experiences with my engine. It could be unique compared to anyone else's. I used valvolene vr1 10w30 for the longest time and it worked great. However I'm in Houston where temperatures commonly exceed 110 in mid summer! At wot oil pressure was fine but I just wasn't comfortable with how low it was at idle, I'm talking right at 10 psi maybe a little lower. Not a big deal but I switched to brad penn 15w40 to get it up slightly, plus brad penn oil has a more ppm zddp additive so I figured it woul help with wear on the old tappet cams. I would even run the 20w50 but I wouldn't want it during cold weather